The proposed research will address three distinct but interconnected biological problems, namely (i) the "gene-number paradox" (i.e., the discrepancy between classical genetic and molecular estimates of the numbers of genes) and related questions about the organization, complexity, and evolution of the eukaryotic genome; (ii) the genetic control of the cell cycle; and (iii) the molecular basis of cellular morphogenesis. These problems will be explored by pursuing four distinct but interconnected lines of investigation, nearly all of which will employ the genetically tractable eukaryotic microorganisms Saccharomyces cerevisiae and Schizosaccharomyces pombe, as follows. First, intensive molecular genetic studies of S. cerevisiae chromosome I and genes derived therefrom will be continued. The distribution of essential and nonessential genes will be determined; an attempt will be made to generate temperature-sensitive mutations in essential genes that were refractory to such mutations during in vivo mutant hunts; and a plasmid-sectoring assay will be used to attempt to determine whether nonessential genes are so because they are functionally redundant. Second, genetic and biochemical studies of the 10-nm filaments of S. cerevisiae (an apparently novel cytoskeletal system) will be continued. Additional filament-associated proteins and possible posttransitional regulators of these proteins will be sought, the possible membrane association of the filaments will be investigated, and filament assembly in vitro will be attempted. In addition, apparently homologous systems in S. pombe and Drosophila will be studied further, and homologues will also be sought in other organisms. Third, genetic and biochemical studies of the establishment of cell polarity will be continued. The intracellular localization of relevant proteins and the components of a Ras-like protein system will be studied in S. cerevisiae, and an attempt will be made to develop a semi-intact cell system for the analysis of bud-site establishment. Studies of the establishment of growth polarity in S. pombe will be initiated by seeking homologues of the relevant proteins of S. cerevisiae. Fourth, both familiar and novel types of morphogenetic mutants will be sough using novel genetic screens (including a conditional transposon- silencer system), and the most interesting of the new genes identified will be studied at the molecular level.